Melt flow is an important attribute of polycarbonate materials, in view of their relatively high melt viscosity. The high viscosity necessitates the use of high extrusion and molding temperatures when processing the materials. A great deal of effort has been directed toward reducing the melt viscosity of polycarbonates. For example, plasticizers have been incorporated into the polymer systems, or the molecular weight of the polymer has been reduced in some instances. The melt flow characteristic is directly related to its glass transition temperature (Tg). In other words, a decrease in Tg results in a desirable decrease in melt viscosity.
Polyestercarbonates exemplify another very useful technique for reducing the melt flow of polycarbonates. These materials (which are sometimes referred to as "copolyestercarbonates") can be considered as members of the family of polycarbonates, utilizing two of the same starting materials: a dihydric phenol and a carbonate precursor. The polyestercarbonates also contain an aliphatic chain segment of varying length, incorporated by the use of a suitable precursor material, such as an aliphatic dicarboxylic acid. The aliphatic segment increases the melt-flow of the polymer. Polyester-carbonates are described in a variety of references, such as the following U.S. Pat. Nos. 5,274,068 (Boden et al); 5,025,081 (Fontana et al); 4,983,706 (Fontana et al); and 4,286,083 (Kochanowski).
Certain types of monomers for supplying the aliphatic ester units for the polyestercarbonate are usually preferred. For example, U.S. Pat. No. 4,983,706 describes the use of aliphatic alpha omega dicarboxylic acids containing from 8 to about 20 carbon atoms, and preferably, about 9 or 10 carbon atoms. Moreover, saturated acids are said to be preferred. In U.S. Pat. No. 5,274,068, the use of aliphatic diacids having from 4 to 8 carbon atoms (and preferably 6 atoms) is disclosed, with adipic acid being the preferred monomer. Furthermore, U.S. Pat. No. 4,286,083 describes the use of saturated, aliphatic dibasic acids derived from straight chain paraffin hydrocarbons, such as oxalic, malonic, dimethyl malonic, succinic, glutaric, adipic, pimelic, and the like. Two specific unsaturated acids are also mentioned for possible use: maleic acid and fumaric acid, each of which contains only 4 carbon atoms.
Clearly, there has been some success in reducing the melt viscosity of polycarbonate-based materials like the polyestercarbonates. However, further reductions in viscosity would be welcome. Even a small reduction in the Tg of the polymer would result in a very desirable increase in melt flow. Thus, a need still exists for polycarbonate-based materials with Tg's which are lower than those of the presently-available products. Moreover, the materials should still retain the other characteristics which make them attractive for a number of end uses--characteristics such as toughness, clarity, rigidity, and high impact resistance.